The present invention relates to well logging, and in particular to improvements in a borehole logging tool referred to as a borehole televiewer, or BHTV. Tools of this type are described, for example, in U.S. Pat. Nos. 3,369,626 (Zemanek, Jr., issued Feb. 20, 1968), 3,478,839 (Zemanek, Jr., issued Nov. 18, 1969), 4,463,378 (Rambow, issued July 31, 1984), and 4,601,024 (Broding, issued July 15, 1986).
In general, borehole televiewer logging tools operate acoustically by periodically pulsing a rotating acoustic transducer to emit a sequence of acoustical pulses directionally into the borehole toward the borehole wall, and analyzing the echos which are reflected back to the tool. The amplitude of the reflected signal may then be displayed on a cathode ray tube, the display sometimes being photographed for future reference. Typically, the display represents a map of the borehole wall split along the north direction and laid out flat. Additionally, a polar display may be produced, in which case the radius of the circular trace is determined by the time-of-flight of the acoustic pulse, thus presenting a cross-sectional profile of the borehole. Another display, similar to the amplitude display, is modulated by the time-of-flight signal rather than the amplitude signal. The latter can be converted into a pseudo-three-dimensional image by adding a slight bias to the vertical sweep according to the magnitude of the time-of-flight signal. BHTV tools typically include means for monitoring the tool orientation within the borehole, such as a fluxgate magnetometer rotating in unison with the transducer. A good technical description of a borehole televiewer suitable for use in a geothermal environment may be found in "Development of a Geothermal Acoustic Borehole Televiewer", by Fred B. Heard and Tom J. Bauman, Sandia Report SAND83-0681, August 1983.
One of the principal and extremely valuable benefits furnished by the BHTV logging tool is the pseudo "visual" image of the borehole wall which it furnishes. Subtleties in the formation, bedding, bedding planes, dip, and so forth, can be observed and studied in a manner unavailable elsewhere. Especially in the oil industry, conventional optical viewing devices do not suffice, in part due to the typically extremely hostile environment, but primarily because the fluid medium in the borehole is normally opaque to optical energy.
As shown in the above-noted publications, borehole televiewers scan radially, typically with a single transducer, essentially looking at a small ring encircling the transducer in the transverse plane thereof. As the borehole televiewer is then moved vertically through the borehole, the path or trail of this ring, as it moves along the borehole wall, in turn describes the wall. This description is then accumulated to generate the displays discussed above.
Not only is the fluid medium in a typical borehole opaque to optical energy, it is usually quite attenuating to acoustical energy as well. That is, in order to contain the elevated pressures commonly encountered in hydrocarbon bearing formations, additives are mixed into the fluid in the borehole. This slurry (referred to as "mud" in the industry) has a density which is adjusted, either by mixing in more additive or by diluting with more water, as needed for the conditions at hand. Unfortunately, the mud in the borehole acts as an acoustic attenuator, reducing the amplitude of the signal as the mud transit distance increases. A typical mud attenuation factor is about 1db/inch/percent of suspended solids in fresh water muds. Since typical borehole diameters are only a few inches greater than the diameter of the borehole televiewer, it will be readily appreciated that even rather small changes in borehole diameter can seriously impact the quality of the image being generated. Of course, not only are uneven borehole walls (e.g., washed out areas, vuggy areas, etc.) negatively impacted, but intentional changes in the borehole size (such as where the size of the drill bit was changed) as well as zones where the borehole was drilled very large to begin with, can drive a televiewer out of its operational capability, especially in heavily attenuating muds. This is illustrated figuratively in FIGS. 2A and 2B, the former showing the effects of mud attenuation on reflected signal amplitude in an 8-inch borehole, and the latter showing the trace which would be seen in a 10-inch borehole. Observe as well that, not only is the reflection signal increasingly attenuated with distance, but there is also a significant difference in amplitude between the initial or "fire" pulse and the reflection or signal pulse.
In the typical prior art operational regime, the receiver amplifier for the transducer is turned on just after the fire pulse. In an ideal system, nothing would be heard by the amplifier until the reflection signal returned from the borehole wall. Unfortunately, this is not what actually happens. In actual practice, there are reflections at the borehole televiewer window, and, particularly in heavy muds, large amounts of signal scattering. The former causes a problem known as ringdown of the window/transducer system. The latter causes false echos.
One solution to these problems is to keep the amplification level rather low during the initial period, and then to step it up to a new, much higher level just before the expected arrival of the return or reflection signal. While a definite step in the right direction, this solution still fails to compensate dynamically for the borehole attentuation of the signals discussed earlier. Where there are differences in the mud-induced attenuation of the signal, time-dependent borehole attenuation of the detected acoustic energy signals will still cause distortion or degradation of the acoustic image generated therefrom.
A need therefore remains for an improved borehole imaging apparatus and method which can compensate not only for initial ringdown and mud reflections, but also for mud attenuation and time-dependent borehole attenuation of the detected borehole reflection acoustic energy signals. Preferably, such a method and apparatus would be dynamically responsive, on a pulse-by-pulse basis, usable over the widest possible range of borehole conditions, adjustably controllable from the surface for precisely responding to this wide range of conditions, and easy to implement in an inexpensive, uncomplicated, highly versatile, efficient, and reliable method and apparatus for the greatest possible utilization in borehole imaging.